Chapter 5 Arrays

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Chapter 5 Arrays
 Introducing Arrays
 Declaring Array
Variables, Creating Arrays,
and Initializing Arrays
 Passing Arrays to Methods
 Copying Arrays
 Multidimensional Arrays
 Search and Sorting Methods
Introducing Arrays
Array is a data structure that represents a collection of the
same types of data.
double[] myList = new double[10];
myList
reference
myList[0]
myList[1]
myList[2]
myList[3]
myList[4]
myList[5]
myList[6]
myList[7]
myList[8]
myList[9]
An Array of 10
Elements
of type double
Declaring Array Variables

datatype[] arrayname;
Example:
double[] myList;

datatype arrayname[];
Example:
double myList[];
Creating Arrays
arrayName = new datatype[arraySize];
Example:
myList = new double[10];
references the first element in the array.
myList[9] references the last element in the array.
myList[0]
Declaring and Creating
in One Step

datatype[] arrayname = new
datatype[arraySize];
double[] myList = new double[10];

datatype arrayname[] = new
datatype[arraySize];
double myList[] = new double[10];
The Length of Arrays
 Once
an array is created, its size is fixed. It
cannot be changed. You can find its size using
arrayVariable.length
For example,
myList.length returns 10
Initializing Arrays
 Using
a loop:
for (int i = 0; i < myList.length; i++)
myList[i] = i;
 Declaring,
creating, initializing in one step:
double[] myList = {1.9, 2.9, 3.4, 3.5};
This shorthand syntax must be in one statement.
Declaring, creating, initializing
Using the Shorthand Notation
double[] myList = {1.9, 2.9, 3.4, 3.5};
This shorthand notation is equivalent to the
following statements:
double[] myList = new double[4];
myList[0] = 1.9;
myList[1] = 2.9;
myList[2] = 3.4;
myList[3] = 3.5;
CAUTION
Using the shorthand notation,
you have to declare, create,
and initialize the array all
in one statement. Splitting it
would cause a syntax error.
For example, the following is
wrong:
double[] myList;
myList = {1.9, 2.9, 3.4, 3.5};
Example 5.1
Testing Arrays

Objective: The program receives 6 numbers from
the keyboard, finds the largest number
and counts the occurrence of the
largest number entered from the
keyboard.
Suppose you entered 3, 5, 2, 5,
5, and 5, the largest number is 5
and its occurrence count is 4.
TestArray
Run
Example 5.2
Assigning Grades

Objective: read student scores (int) from the
keyboard, get the best score, and then assign
grades based on the following scheme:
– Grade is A if score is >= best–10;
– Grade is B if score is >= best–20;
– Grade is C if score is >= best–30;
– Grade is D if score is >= best–40;
– Grade is F otherwise.
AssignGrade
Run
Passing Arrays to Methods
Java uses pass by value to pass
parameters to a method. There are
important differences between passing
a value of variables of primitive
data types and passing arrays.
 For a parameter of a primitive type
value, the actual value is passed.
Changing the value of the local
parameter inside the method does not
affect the value of the variable
outside the method.
 For a parameter of an array type,
the value of the parameter contains a
Example 5.3
Passing Arrays as Arguments
 Objective:
Demonstrate differences of
passing primitive data type variables
and array variables.
TestPassArray
Run
Example 5.3, cont.
swap(a[0], a[1])
a[0]
1
a[1]
2
n1
1
n2
2
Pass by value
swap( n1,
n2)
swapFirstTwoInArray(a)
a Reference
:
Pass by value (Reference value)
swapFirstTwoInArray(array)
array Reference
a[0]
a[1]
Example 5.4 Computing Deviation
Using Arrays
n
m ean 
 xi
i 1
n
Deviation
n
deviation
2
(
x

m
ean
)
 i
i 1
n 1
Run
Example 5.5
Counting Occurrence of Each Letter
 Generate
100 lowercase letters randomly and
assign to an array of characters.
 Count the occurrence of each letter in the
array.
 Find the mean and standard deviation of the
counts.
CountLettersInArray
Run
Example 5.6
Copying Arrays
In this example, you will see that a simple
assignment cannot copy arrays in the
following program. The program simply
creates two arrays and attempts to copy one to
the other, using an assignment statement.
TestCopyArray
Run
Copying Arrays
Before the assignment
list2 = list1;
list1
After the assignment
list2 = list1;
Contents
of list1
list2
list1
Contents
of list1
list2
Contents
of list2
Garbage
Contents
of list2
Copying Arrays
Using a loop:
int[] sourceArray = {2, 3, 1, 5, 10};
int[] targetArray = new
int[sourceArray.length];
for (int i = 0; i < sourceArrays.length; i++)
targetArray[i] = sourceArray[i];
The arraycopy Utility
arraycopy(sourceArray, src_pos,
targetArray, tar_pos, length);
Example:
System.arraycopy(sourceArray, 0,
targetArray, 0, sourceArray.length);
Multidimensional Arrays
Declaring Variables of Multidimensional Arrays and
Creating Multidimensional Arrays
int[][] matrix = new int[10][10];
or
int matrix[][] = new int[10][10];
matrix[0][0] = 3;
for (int i=0; i<matrix.length; i++)
for (int j=0; j<matrix[i].length; j++)
{
matrix[i][j] = (int)(Math.random()*1000);
}
double[][] x;
Multidimensional Array Illustration
0 1
2
3
4
0 1
2
3
4
0
0
0
0
1
1
1
2
2
2
3
3
4
4
matrix = new int[5][5];
7
matrix[2][1] = 7;
3
1
1
2
2
3
4
5
6
7
8
9
10
11
12
int[][] array = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9},
{10, 11, 12}
};
Declaring, Creating, and Initializing Using
Shorthand Notations
You can also use a shorthand notation to declare, create and
initialize a two-dimensional array. For example,
int[][] array = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9},
{10, 11, 12}
};
This is equivalent to the following statements:
int[][] array
array[0][0] =
array[1][0] =
array[2][0] =
array[3][0] =
= new int[4][3];
1; array[0][1] = 2; array[0][2] =
4; array[1][1] = 5; array[1][2] =
7; array[2][1] = 8; array[2][2] =
10; array[3][1] = 11; array[3][2]
3;
6;
9;
= 12;
Lengths of Multidimensional
Arrays
int[][] array = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9},
{10, 11, 12}
};
array.length
array[0].length
array[1].length
array[2].length
Ragged Arrays
Each row in a two-dimensional array is
itself an array. So, the rows can have
different lengths. Such an array is
known as a ragged array. For example,
int[][] matrix = {
{1, 2, 3, 4, 5},
{2, 3, 4, 5},
{3, 4, 5},
{4, 5},
{5}
};
Example 5.7
Adding and Multiplying Two Matrices
 Objective:
Use two-dimensional arrays to
create two matrices, and then add and multiply
the two matrices.
 a11 a12 a13 a14 a15 
 b11 b12 b13 b14 b15 
 a11  b11





a
21 a 22 a 23 a 24 a 25
b
21 b 22 b 23 b 24 b 25




 a 21  b21
 a 31 a 32 a 33 a 34 a 35    b31 b32 b33 b34 b35    a 31  b31





 a 41 a 42 a 43 a 44 a 45 
 b41 b42 b43 b44 b45 
 a 41  b41
 a 51 a 52 a 53 a 54 a 55 
 b51 b52 b53 b54 b55 
 a 51  b51





a12  b12 a13  b13 a14  b14 a15  b15 

a 22  b22 a 23  b23 a 24  b24 a 25  b25 
a 32  b32 a 33  b33 a 34  b34 a 35  b35 

a 42  b42 a 43  b43 a 44  b44 a 45  b45 
a 52  b52 a 53  b53 a 54  b54 a 55  b55 
TestMatrixOperation
Run
Example 5.7 (cont) Adding and
Multiplying Two Matrices
 a11 a12 a13 a14 a15 
 b11 b12 b13 b14 b15 
 c11 c12 c13 c14 c15 






 a 21 a 22 a 23 a 24 a 25 
 b21 b22 b23 b24 b25 
 c 21 c 22 c 23 c 24 c 25 
 a 31 a 32 a 33 a 34 a 35    b31 b32 b33 b34 b35    c 31 c 32 c 33 c 34 c 35 






 a 41 a 42 a 43 a 44 a 45 
 b41 b42 b43 b44 b45 
 c 41 c 42 c 43 c 44 c 45 
 a 51 a 52 a 53 a 54 a 55 
 b51 b52 b53 b54 b55 
 c 51 c 52 c 53 c 54 c 55 






cij = ai1b1j+ai2b2j+ai3b3j+ai4b4j+ai5b5j
Example 5.8
Grading Multiple-Choice Test

Students’ Answers to the Questions:
0 1 2 3 4 5 6 7 8 9
Student
Student
Student
Student
Student
Student
Student
Student
0
1
2
3
4
5
6
7
A
D
E
C
A
B
B
E
B
B
D
B
B
B
B
B
A
A
D
A
D
E
A
E
C
B
A
E
C
C
C
C
C
C
C
D
C
C
C
C
D
A
B
C
D
D
D
D
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
A
A
A
A
A
A
A
A
D
D
D
D
D
D
D
D
Objective: write a
program that grades
multiple-choice test.
Key to the Questions:
0 1 2 3 4 5 6 7 8 9
Key
D B D C C D A E A D
Grade Exam
Run
Example 5.9
Calculating Total Scores

Objective: write a program that calculates the total score for
students in a class. Suppose the scores are stored in a threedimensional array named scores. The first index in scores refers to
a student, the second refers to an exam, and the third refers to the
part of the exam. Suppose there are 7 students, 5 exams, and each
exam has two parts--the multiple-choice part and the programming
part. So, scores[i][j][0] represents the score on the multiple-choice
part for the i’s student on the j’s exam. Your program displays the
total score for each student, .
TotalScore
Run
Searching Arrays
Searching is the process of
looking for a specific element
in an array; for example,
discovering whether a certain
score is included in a list of
scores. Searching, like
sorting, is a common task in
computer programming. There are
many algorithms and data
structures devoted to
searching. In this section, two
Linear Search
The linear search approach
compares the key element, key,
with each element in the array
list[]. The method continues to
do so until the key matches an
element in the list or the list
is exhausted without a match
being found. If a match is
made, the linear search returns
the index of the element in the
array that matches the key. If
Example 5.10
Testing Linear Search
 Objective:
Implement and test the linear search
method by creating an array of 10 elements of
int type randomly and then display this array.
Prompt the user to enter a key for testing the
linear search.
LinearSearch
Run
Binary Search
For binary search to work, the
elements in the array must
already be ordered. Without
loss of generality, assume that
the array is in ascending
order.
e.g. 2 4 7 10 11 45 50 59 60 66
69 70 79
The binary search first
compares the key with the
Binary Search, cont.
 If the key is less than the
middle element, you only need
to search the key in the first
half of the array.
 If the key is equal to the
middle element, the search ends
with a match.
 If the key is greater than the
middle element, you only need
to search the key in the second
Binary Search, cont.
key = 11
[0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
list
2
4
7
10 11 45
key < 50
mid
[0] [1] [2] [3] [4] [5]
2
key > 7
4
7
10 11 45
mid
[3] [4] [5]
10 11 45
key = 11
50 59 60 66 69 70 79
mid
Example 5.11
Testing Binary Search
Implement and test the
binary search method. The
program first creates an array
of 10 elements of int type. It
displays this array and then
prompts the user to enter a key
for testing binary search.
 Objective:
BinarySearch
Run
Example 5.12
Using Arrays in Sorting

Objective: Use the selectionSort method to write a
program that will sort a list of double floating-point
numbers.
int[] myList = {2, 9, 5, 4, 8, 1, 6}; // Unsorted
Sort it to produce 1, 2, 4, 5, 6, 8, 9
2, 9, 5, 4, 8, 1, 6
SelectionSort
Run
Example 5.12: (Cont) Using
Arrays in Sorting
int[] myList = {2, 9, 5, 4, 8, 1, 6}; // Unsorted
Find the largest element in myList and swap it with the last
element in myList.
2, 9, 5, 4, 8, 1, 6 => 2, 6, 5, 4, 8, 1, 9 (size = 7)
2, 6, 5, 4, 8, 1 => 2, 6, 5, 4, 1, 8 (size = 6)
2, 6, 5, 4, 1 => 2, 1, 5, 4, 6 (size = 5)
2, 1, 5, 4 => 2, 1, 4, 5
2, 1, 4 => 2, 1, 4,
2, 1 => 1, 2
Sort it to produce 1, 2, 4, 5, 6, 8, 9
Exercise 5.5: Bubble Sort
int[] myList = {2, 9, 5, 4, 8, 1, 6}; // Unsorted
Pass 1:
Pass 2:
Pass 3:
Pass 4:
Pass 5:
Pass 6:
2, 5, 4, 8, 1, 6, 9
2, 4, 5, 1, 6, 8, 9
2, 4, 1, 5, 6, 8, 9
2, 1, 4, 5, 6, 8, 9
1, 2, 4, 5, 6, 8, 9
1, 2, 4, 5, 6, 8, 9

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